Legal regulations which are becoming ever stricter with regard to permissible pollutant emissions of internal combustion engines for motor vehicles make it necessary to take measures, by way of which the pollutant emissions can be lowered. It is a starting point here to achieve improved mixture preparation in the cylinders of the internal combustion engine. Correspondingly improved mixture preparation can be achieved if fuel is metered in at a defined pressure by means of fuel injectors. In the case of a diesel internal combustion engine, fuel pressures of this type are as high as over 2000 bar.
In a fuel injector, control of an injection of fuel usually takes place by means of a nozzle needle which is mounted displaceably in a nozzle assembly of the fuel injector and releases or closes one or a plurality of spray holes of a nozzle body of the nozzle assembly for the fuel to be injected as a function of its position. A mechanical actuation of the nozzle needle usually takes place by way of an actuator, preferably a piezoelectric actuator, which either acts mechanically with the nozzle needle or acts via a servovalve and a control space on a transmission element (piston) which interacts mechanically with the nozzle needle or is formed integrally with the latter. The nozzle needle and the transmission element are usually mounted slidingly here in a sliding guide with a small play, lubrication of this mounting as a rule taking place by way of the fuel to be injected.
In order to lower the pollutant emissions and also to keep a consumption of the internal combustion engine as low as possible, it is desirable to achieve as optimum as possible a combustion within the cylinders of the internal combustion engine. For satisfactory process management and control/regulation of a combustion in the cylinders of the internal combustion engine, it is necessary for it to be possible to meter the fuel to be injected as accurately as possible, in order at every instant to achieve as optimum as possible a combustion and/or as complete as possible a regeneration of a particle filter.
Torque requirements of the internal combustion engine are converted into injection quantities. Each injection quantity is correlated with an injection time as a function of an injection pressure. The resulting injection characteristic curves are stored as a nominal injection characteristic diagram (see also FIG. 1) in software of a controller for the internal combustion engine. These correlations are used for all fuel injectors, individual differences of the fuel injectors, caused, for example, by production deviations or ageing and wear of the components, not being taken into consideration during the entire service life of the fuel injectors.
Deviations of the actual injection quantities from the setpoint injection quantities (see also FIG. 2), the latter being called nominal injection quantities in the following text, always have negative effects on a combustion and the pollutant emissions which are produced as a result. If the injection quantities are too small and the actuating times of the fuel injectors are therefore too short, failure of injections and therefore uneven running of the relevant internal combustion engine can occur, moreover. If the injection quantities of the fuel injectors are too great and/or their actuating times are too long, overheating of the internal combustion engine can be the result.
For these reasons, an individual adaptation of the injection quantities and/or times of the relevant fuel injectors is desirable. That is to say, the injection quantities and/or times of each fuel injector are to be adapted to the nominal injection time and/or injection quantity characteristic diagram. This is required, in particular, on account of constantly lowering legal emissions limiting values.
In the prior art (see also below), two methods exist, by way of which an injector-individual adaptation to the nominal injection characteristic diagram is realized partially. This is what is known as IIC (injector individual correction) and MFMA (minimum fuel mass adaptation), MFMA being suitable only for a lower ballistic range of a nozzle needle movement for injection quantities up to approximately 3 mg, and IIC operating too imprecisely in the ballistic range.